The invention relates to detection of defective nuclear fuel elements in an immersed assembly, the elements comprising a tubular metal sleeve closed by impervious caps, containing a stack of pellets of nuclear fuel surrounded by an atmosphere of gas under pressure. An especially important application of the invention is detection and identification of defective fuel elements in an assembly for a nuclear reactor, having a length of several meters, each containing a stack of fuel pellets held in contact by a spring compressed between the stack and an end cap and placed in a plenum occupied by the gas under pressure.
Various methods have already been used for determining whether a spent fuel assembly contains defective elements in which the sleeve is no longer gas tight. The most commonly used approach consists of placing the irradiated assembly removed from the nuclear reactor in a cell where it is heated, so that the pressure of the fission gas contained in the fuel elements increases and the gas escapes into the cell through the cracks in the defective fuel elements. However, this technique does not allow identification of the defective fuel elements to be replaced.
It has also been suggested to apply known techniques of non-destructive ultrasonic testing to inspection of fuel elements, before mounting in place in their assemblies, and to detection of defective fuel elements after irradiation. Many of these methods are applicable only to a separate fuel element while it is not immersed, which constitutes a troublesome limitation. Among these methods there may be mentioned heating of the expansion chamber followed by ultrasonic detection of water condensed on the cap (French Patent Applications Nos. 2 222 732 and 2 365 185) and measurement of the attenuation, caused by water which may be contained in the element, of an ultrasonic wave transmitted in the sheath (French Patent Applications Nos. 2 287 753 and 2 341 182). This line of thought in the prior art may be attributed to the fact that it has been attempted to preserve the measurement from the effect of the surrounding medium. However, there have also been suggested methods of detecting defective fuel elements while these are immersed. The methods suggested up to now only relate to detection of one indication representative of a fault, in particular the presence of water in the sheath. For example, it has been suggested to detect the presence of liquid water in the lower portion of each element in the expansion chamber, by directing an ultrasonic beam transversely to the element (French Patent Application No. 2 341 183). A preferred method (French Patent Application No. 2 493 025) consists of detecting the ultrasonic energy diffused in the liquid body by a fault present in an element in which the ultrasonic waves travel.
It is an object of the invention to improve upon the prior art methods for detecting defective fuel elements. It is a more specific object to provide a method which is responsive to various features indicative of a fault, particularly both to entry of water in the sheath and to the presence of cracks or flaws therein. For achieving that result, the inventors had to appreciate a number of factors which had been neglected or overlooked. The first is that ultrasonic waves having a frequency such that they travel in the "plate" mode or Lamb mode, already suggested for inspection of fuel elements which are new and not immersed (French Patent Application No. 2 454 675) are not appreciably attenuated by a liquid surrounding the outer surface of the sheath. A second discovery, also important, is that the simultaneous presence of pellets of fuel and a film of water surrounding them in the sheath causes a very large attenuation of the Lamb waves in the S.sub.o mode whereas it was known that there would be hardly any attenuation in a sheath containing no fuel, but containing water, even in a large amount.
A method according to the invention for detecting defective nuclear fuel elements in an assembly immersed in liquid using ultrasonic absorption includes the steps of transmission in the sheath, from an end part of the latter, of a signal comprising a train of ultrasonic waves having a frequency and duration chosen such that propagation takes place as Lamb waves, and detection of echoes; and repetition of transmission and reception at different frequencies, situated in a range of frequencies part of which corresponds to substantial absorption by water located in the sheath and part of which corresponds to a major formation of echo at a structural fault in the sheath.
To cause propagation in the Lamb mode in a sheath, the ultrasonic sound waves shall be applied in the form of a train, with a frequency such that the wavelength is equal to or less than the thickness of the sheath. The upper limit of the acceptable frequency range is therefore easily determined. Within the range thus defined, a first range of frequencies is selected which corresponds to maximum absorption responsive to presence of a film of water between the pellets and the sheath and a second range is selected which corresponds to detection of mechanical faults in the sheath. In practice, there is used for the detection of the presence of liquid a first range of frequencies f.sub.1 less than the range of frequencies f.sub.2 for detection of mechanical faults. The ranges f.sub.1 and f.sub.2 will correspond as a general rule to a frequency less than 1 megahertz, that is to say considerably less than the frequencies now used for ultrasonic inspection of metal articles.
In carrying out later an analysis of the return echoes of the trains of ultrasonic waves applied to the spring through the end cap, trains of a frequency and duration such that there is propagation in the Lamb mode of type A.sub.o or A.sub.1 in the spring, there is obtained an indication of the gas pressure prevailing in the expansion chamber and surrounding the spring. The range of frequencies corresponding to the latter determination is generally considerably less than f.sub.1 and f.sub.2.
It is advantageous to carry out inspection of a rod in a single sequence of operations, by application of a single transducer, coupled to an end cap or to the sheath, used as a transmitter and also as a detector, for trains of waves of which the frequency is progressively varied to sweep the ranges f.sub.1, f.sub.2 and f.sub.3. In each of these preselected ranges it is possible either to increase the maximum value of the echoes or, better, to take an average of the echoes over the whole of the preselected range. By sampling and digitisation, it is also possible to arrange the information from the tests and to apply to them any mathematical treatment intended to produce the useful information and to reduce the base noise.
The invention also relates to a device for detecting defective fuel elements in a nuclear feul assembly, each of the elements comprising an impervious sheath containing a stack of pellets of fuel surrounded by an atmosphere of gas under pressure, which device is characterized in that it comprises: an electro-acoustic transducer which can apply trains of ultrasonic waves in the Lamb mode to the sheath; a generator for exciting the transducer with trains of waves having different frequencies in ranges corresponding to propagation in the Lamb mode and, for one of the ranges, to considerable adsorption in the event of the presence of liquid in the sheath; and circuits for reception and treatment of the echoes.
It is possible, by calculation, using the usual criteria, for example those explained by Voktorov in "Rayleigh and Lamb waves", to determine approximately the ranges of frequency corresponding to propagation in the S mode (symmetric) and A mode (anti-symmetric) for a given thickness of sheath, dimension of spring and a given type of material. However, only experiment allows determination, for each rod, of the optimum frequency. As this frequency may vary from rod to rod because of tolerances of the latter, it suffices to use a range of frequencies covering the optimum values for the group of rods and to take for each of them, a mean of the echoes obtained at the different frequencies in the range.
The invention will be better understood from the following description and particular embodiments given by way of non-limiting example.